Magnetic disks contain data written inside concentric circular tracks. They take the form of a succession of small magnetic cells, having a generally variable length, distributed over the entire length of each track. The magnetization in two consecutive elementary magnetic cells is in opposite directions and generally has the same modulus.
The means which enables data to be either written on the magnetic disks or read therefrom, or which enables both of these functions to be realized, is known as a transducer.
Magnetic transducers comprising a magnetic circuit and a winding around said circuit are disposed are commonly used for both writing and reading. The magnetic circuit generally includes two pole pieces separated by an air gap. This circuit is placed a very slight distance (between 0 and a few tenths of a micron) away from the face of a magnetic data carrier. The pole piece that is chronologically the first one to have the data on the magnetic data carrier rotate past it is called the upstream, or front, pole piece, while the other pole piece is called the downstream, or rear, pole piece.
The trend in present-day development of magnetic disks is to seek ways of attaining radial densities of several thousand tracks per centimeter (measured along the diameter of the disks), and linear densities equal to or greater than 10,000 changes in magnetization direction per centimeter (measured along the circumference of the tracks).
One of the methods of writing data that enables such recording densities to be attained is known as transverse writing. In this method, the magnetization in the elementary cells is located in the plane of the magnetic recording film of the disk (that is, parallel to the disk) and perpendicular to the direction in which the data are moved in the recording process, hence the term transverse recording. In this mode, the magnetic medium comprising the film is preferably an anisotropic magnetic medium having one preferred direction of magnetization, known as the direction of easy magnetization, contained in the plane of the recording film and perpendicular to the direction in which the data are moved in the recording process. For a magnetic disk, this direction is radial in any case, and the magnetization in the cells is therefore oriented along a diameter of the disk.
Each elementary cell of a given track on the recording carrier is separated from each of its neighbors by a magnetic transition (in fact, a Bloch or Neel wall, a geometric locus of points separating two adjacent elementary cells of opposite magnetization, indicating the reversal of the direction of magnetization between these cells), which is known as the principal transition. Said principal transition is oriented along a diameter of the disk, perpendicular to the direction in which the data rotate. On the data carrier, each principal transition physically defines a binary datum.
More generally, a magnetic transition is defined as the geographic border between magnetic cells of opposite magnetization.
To write data using the transverse writing method, a recording system comprising a magnetic transducer is typically used, with the air gap of its magnetic circuit aligned perpendicularly to the direction in which the data rotate and to the surface of a recording carrier of the type described above (anisotropic magnetic disk with a radial direction of easy magnetization, for example).
The magnetic writing field used is the component of the magnetic field produced by the rear pole piece, that is, the component that is parallel to the recording carrier and perpendicular to the direction in which the data rotate.
It has been found that the system and the recording method described above makes it possible to obtain magnetic transitions separating the successive magnetic cells of each track which are very precisely defined and are highly stable. It has also been found that the length of each transition (measured parallel to the direction of rotation of the data) decreases as the thickness of the recording film decreases.
In order for data recorded by the transverse method to be readable, each of the magnetic cells must emit a stray magnetic field, in the immediate vicinity of the recording carrier, of sufficient intensity that a reading transducer will furnish a signal that is usable by the electronic reading circuits associated with it.
By the nature of transverse recording, however, there is no stray magnetic field produced vertically, with respect to the principal transitions, in the vicinity of the surface of the data carrier. Indeed, the lines of the magnetic field between two adjacent cells about these transitions have the tendency to close within the same plane as the data carrier, rather than outside it.
In order for the magnetic cells of a track to produce a stray magnetic field in the vicinity of the surface of the data carrier making it possible to detect its location, each of these cells must include a lateral transition on each of its sides, that is, transitions parallel to the direction in which the data rotate. This means that a cell of a given track must be surrounded, on both sides of the track, by magnetic cells which possess magnetization opposite to that of the cell in question.
In other words, only the magnetic cells that are provided with two lateral transitions, arranged in the way described above, will produce, in the immediate vicinity of the recording carrier, a stray magnetic field having sufficient intensity to produce a reading signal that is usable by the terminals of a magnetoresistant reading transducer.
Thus, for the magnetic cells of a given track, it is the principal transitions that make it possible to define the geographic location of the binary data on the disk, and it is the lateral transitions that enable these data to be detected by a reading transductor.
The disadvantage of the system and of the typical recording methods described above is that mapping the stray magnetic field generated by the recorded data is heavily dependent on the initial magnetic state of the track to be used for recording, and of its immediate magnetic environment (this term being defined as a zone of limited size which is located on both sides of the edges of the track).
In fact, the initial magnetic state of the track and its immediate magnetic environment is quite random, which may have one of the following consequences:
(a) a recorded magnetic cell is provided with a lateral transition on each of its sides that are parallel to the direction of data rotation, in which case the output signal of the reading transducer is usable;
(b) the recorded magnetic cell is provided with a lateral transition on only one of its sides, in which case the stray magnetic field that is produced is relatively weak, and the output signal of the magnetoresistant reading transducer is weak, therefore, hard to use;
(c) the recorded magnetic domain does not include lateral transitions on either side in which case the stray magnetic field that is produced is virtually zero, and the output signal of the magnetoresistant reading transducer is accordingly zero as well.
The set of reading signals to be read corresponding to a given track will accordingly include two types of signals:
(1) signals having enough amplitude that they enable a determination of the value of the corresponding data (i.e., case above);
(2) signals with weak or even zero amplitude, making it either uncertain or impossible to determine the value of the corresponding data (cases (b) and (c) above). The signal corresponding to the set of recorded magnetic domains on a given track will, therefore, be extremely difficult to use.